Problem: Simplify the following expression: $p = \dfrac{10}{9a} - \dfrac{7}{6a}$
In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9a$ and $6a$ $\lcm(9a, 6a) = 18a$ $ p = \dfrac{2}{2} \cdot \dfrac{10}{9a} - \dfrac{3}{3} \cdot \dfrac{7}{6a} $ $p = \dfrac{20}{18a} - \dfrac{21}{18a}$ $p = \dfrac{20 -21}{18a}$ $p = \dfrac{-1}{18a}$